Mark,  In the lock and cancel arrangement on Wurlitzer, the two block
valves, ideally should be a close match _and_ they must be in perfect
condition.

I feel your problem must be inside the cancel valve itself and would be
looking at a partially blocked bleed or a bad pouch, that stops the
valve from working with normal signal strength.

If the trouble came from a loose nipple causing a leak, the valve would
drop back into play mode at any time during the rewind.  Your problem
is the reverse.  The signal from when the finger on the take up spool
gets tripped, is not strong enough to overcome the suction in the
cancel valve pouch -- but when you pull off the tube, it makes just
enough difference that it will works as intended.

If you are certain that there is no obstruction in the finger valve
unit or the tube leading to the cancel valve, then look at the cancel
valve itself.

Last thought, if you have old lead tubing from the finger valve on the
take up spool, to the cancel valve which throws everything from rewind
back into play, that can also be a source of trouble should this tube
have lead oxide disease OR if it may have been accidentally crushed or
crimped at some time as that can also restrict the flow of atmosphere
to the cancel valve.

Using lock and cancel is a fast and good system instead of the more
mechanical way of a latching finger, but there is no forgiveness in
the operation of the valves.  They must be in top shape.

Ken Vinen
Aylmer, Ontario, Canada